Simulation example: Single Sideband transceiver

This tutorial explains how a Single Sideband (SSB) signal can be generated and received. Rather than using any real hardware for transmission, the signal is sent via a socket from the transmit section to the receive section. The only actual hardware involved is the computer's microphone input and speaker output. In the case of a Raspberry Pi computer, which has no microphone input, two alternatives are presented.

This tutorial can be performed with either GNU Radio (GR) version 3.7 or 3.8 (and later). The Graphical User Interface gnuradio-companion (GRC) is used to create a flowgraph for each section.

Prerequisites

• Intro to GR usage: GRC and flowgraphs
• Understanding sample rate

SSB receiver

The Weaver method of SSB demodulation

Traditionally, SSB has been created and received using narrow bandpass filters, or by using a "phasing method" requiring a 90 degree phase shift network across the audio spectrum. In 1956 Donald Weaver published a third method in the Proceedings of the IRE ("A Third Method of Generating and Detecting Single Side Band Signals", Dec. 1956). It involves the use of a local oscillator with a frequency in the middle of the audio spectrum (1500Hz) for the I and Q signals separately, and then adding or subtracting them to recover the upper or lower sideband audio.

Flowgraph

Using gnuradio-companion (GRC) and the following Block descriptions, build this flowgraph of the receiver section:

Block descriptions

• The Options block identifies the filename for the flowgraph, a title, author, etc.
  • id: SSB_rcv
  • Click on File -> Save As  Use the file name 'SSB_rcv'. the extension '.grc' is added automatically
• The signal from the transmitter section is received by the ZMQ PULL Source. It is a 24khz carrier with a sample rate of 768khz.
  • Address: tcp://127.0.0.1:50301
• A Variable block defines the sample rate.
  • id: samp_rate
  • value: 768000
• Another Variable block defines the decimation factor. Since the Audio Sink (speaker) uses a sample rate of 48khz, we will reduce (decimate) the incoming sample rate by a factor of 16.
  • id: decim
  • value: 16
• A QT GUI Range block defines the tuning for the input signal
  • id: tuning
  • label: Tuning
  • default value: 25500
  • start: 19000
  • stop: 29000
  • step: 100
• The Frequency Xlating FIR Filter performs three functions: frequency translation, filtering, and decimation. See Frequency_Xlating_FIR_Filter for more detail.

• A QT GUI Range block defines the tuning of the local oscillators
  • id: bfo
  • label: Fine tuning
  • default value: 1500
  • start: 0
  • stop: 3000
  • step: 10
  • Widget: choose whatever you like
• A QT GUI Chooser allows selection of the Upper or Lower sideband
• The value of the Sideband selector is used as a multiplier in the Multiply Const block.
  • Constant: reverse
• A QT GUI Range block defines an Audio gain (volume) control
  • id: volume
  • default value: 0.2
  • start: 0
  • stop: 1.0
  • step: 0.05
  • Widget: choose whatever you like
• The value of the volume control is used as a multiplier in the Multiply Const block.
  • Constant: volume
• The speaker output is defined by an Audio Sink block. The parameters are:
  • Sample rate: set to 48khz (use the pull-down)
  • Device name: for most speakers (or headphone jacks) built into the computer, the Device name can be left blank; for other cases, see Audio_Sink#Device_Name

Test receiver section

To test the receiver, generate and run the flowgraph. After a few seconds, a GUI window will open with the Tuning, Fine tuning, and Volume controls, along with the sideband select push buttons. To terminate the process cleanly, click on the 'X' in the upper corner of the GUI rather than using Control-C.

SSB transmitter

Flowgraph

Using gnuradio-companion (GRC) and the following Block descriptions, build this flowgraph (a separate one from the receiver) for the transmitter section:

Block descriptions

• The Options block identifies the filename for the flowgraph, a title, author, etc.
  • id: SSB_xmt_1
  • Click on File -> Save As  Use the file name 'SSB_xmt_1'. the extension '.grc' is added automatically
• The microphone input is defined by an Audio Source block. The parameters are:
  • Sample rate: set to 48khz (use the pull-down)
  • Device name: for most microphone jacks built into the computer, the Device name can be left blank; for other cases, see Audio_Source#Device_Name
• For the remainder of the flowgraph, a sample_rate of 768khz is used. This value was chosen to give the 48khz audio frequency 16 samples per cycle (48000 x 16 = 768000).
  • Variable block
    • id: samp_rate
    • value: 768000
• To boost the 48khz sample rate of the audio input to the 768khz sample rate, a Repeat block with an Interpolation value of 16 is used.
• A QT GUI Range block defines an Audio gain (volume) control
  • id: volume
  • default value: 0.5
  • start: 0
  • stop: 20.0
  • step: 0.1
  • Widget: choose whatever you like
• The value of the volume control is used as a multiplier in the Multiply Const block.
  • Constant: volume
• The carrier signal (in this example is 24khz) is generated by the Signal Source block.
  
  • Sample Rate: samp_rate
  • Frequency: carrier_freq
  • Amplitude: 1.0
• To create a double sideband signal, the carrier signal is multiplied by the audio signal.

The Band Pass Filter creates the Single Sideband signal by passing the frequencies from 24300Hz to 27000Hz and rejecting the others (the filter method). This gives a signal with only one sideband (upper) and no carrier. The audio frequencies of that signal are 300Hz to 3000Hz.

• For a real radio transmitter, the output of the Band Pass Filter would be fed to Radio Frequency (RF) hardware. For this tutorial, we are sending the transmit signal to a ZMQ PUSH Sink data socket connected to the receiver section.
  • Address: tcp://127.0.0.1:50301

Note for Raspberry Pi

Since a Raspberry Pi has no audio input jack, there are two alternatives:

1. use a USB audio dongle.
2. replace the Audio Source block with a Signal Source block (frequency: 600) and a Throttle block to simulate an audio input.

Testing

When using GRC, doing a Generate and/or Run creates a Python file with the same name as the .grc file. You can execute the Python file without running GRC again.

For testing this system we will use two processes, so we will need two terminal windows.

Terminal 1:

• since you just finished building the transmit flowgraph, you can just do a Run. After a few seconds, a GUI window will open with the Volume control and the GUI Frequency Sink.

Terminal 2: Open another terminal window.

• change to whatever directory you used to generate the flowgraph for SSB_rcv
• execute the following command:

   python3 -u SSB_rcv.py

• After a few seconds, a GUI window will open with the Tuning, Fine tuning, and Volume controls, along with the sideband select push buttons.

Speaking into the microphone should show a change in the pattern on the QT GUI Time Sink. The level of modulation can be adjusted with the transmit volume control. You should hear your voice from the speakers. The speaker volume can be adjusted with the receive volume control. Adjust the Fine tuning control until you voice sounds natural to you. Note that very small changes (tens of Hz) will make a difference in how it sounds. Then switch to the lower sideband and hear the results!

To terminate each of the processes cleanly, click on the 'X' in the upper corner of the GUI rather than using Control-C.

What do to next

Now that you have a working system, you can experiment:

• make a receiver using the filter method
• make a transmitter using the Weaver method
• make either using the phasing method. hint: use a Hilbert filter for the 90 degree phase shift